Periodontitis is the second most common disease worldwide and is induced by microorganisms including Treponema denticola. Fibronectin (FN) is a multifunctional adhesive glycoprotein that is distributed in plasma, the extracellular matrix, and on the surface of cells. Bacterial FN-binding proteins play important roles in bacterial adhesion and invasion of host tissues. Previous research has found that T. denticola bound FN which was blocked by anti-FN antibodies. However, the knowledge about FN-binding proteins and their roles in the pathogenesis of T. denticola is very limited. The genome project of T. denticola has identified a new putative fibronectin/fibrinogen-binding protein gene, fbp (Gene ID: 2739776), which is homologous to an N-terminal segment of the prokaryotic FN-binding protein. (FbpA, pfam05833). FbpA is a family of homologous Fn-binding protein gene with over 117 members from different bacteria according to the CDD database. The role of fbp in pathogenesis of T. denticola and the interactions between Fbp and FN has not been investigated. In preliminary studies, we expressed recombinant Fbp in E. coli, and identified that rFbp has FN binding properties. The binding of T. denticola to FN was blocked by rFbp in a concentration dependent manner to a maximum level of 42%. We also make anti-rFbp antibodies to confirm that Fbp is expressed in T. denticola by Western blot. We propose that fbp is an important virulence factor. The research hypothesis is that Fbp contributes to the virulence of T. denticola by mediating interaction with host FN. The purpose of this proposal is to analyze the virulence roles of fbp by comparing the phenotypes of fbp mutant and wildtype strains, and to characterize the functions of Fbp by studying the interactions between Fbp protein and FN. Specifically, in Aim 1 we plan to make isogenic mutants of fbp in T. denticola. We will compare fbp mutant with wildtype T. denticola in FN binding, cell attachment, and virulence in a mouse model. In addition, we will rescue fbp mutants by genetic complementation to confirm the function of fbp. In Aim 2, we will screen a random peptide library to identify the binding site of Fbp for FN and subsequently verify the binding site by site-specific mutagenesis on fbp. In addition we will also design and test the capacity of synthetic Fbp peptide to inhibit the binding of T. denticola to FN. We expect to find a peptide which prevents the adherence of T. denticola to host tissue which may serve as an anti-infection factor. In summary, this is first adhesin mutant of T. denticola and first systemic approach to identity the binding site of Fbp protein. We believe that the proposed project is novel and unique and will help us to understand not only the role of fbp in the pathogenesis of T. denticola, but also in other bacteria with fbp genes. The Long term goal is to understand in depth the mechanism of attachment and invasion of T. denticola to periodontal tissues and to characterize the biological function of the FbpA protein family in order to provide new strategies to prevent bacterial infections.